AN EARTHQUAKE CYCLE ALONG THE SOUTH FLANK OF KILAUEA VOLCANO, HAWAII

The intrusion of magma that led to the beginning of the January 3, 1983, eruption along the middle east rift zone of Kilauea Volcano, Hawaii, resulted in widening of the rift zone and compression of the adjacent portion of the south flank of the volcanic edifice. An earlier rift zone event, and subsequent eruption, on September 12, 1977, probably had a similar effect on the south flank. The compression built up since 1977 was relieved by a M = 6.1 earthquake on June 26. 1989. Horizontal displacements associated with the earthquake, determined by using the Global Positioning System (GPS), indicated seaward movement of the south flank of as much as 300 mm. Vertical displacements, measured by a conventional leveling technique and by GPS, indicated subsidence of as much as 250 mm near the east edge of the aftershock zone. These ground displacements can be modeled by horizontal slip of 1.2 m at an average depth of 6 km. The geodetic moment was 1 X 10(26) dyn cm, a factor of 2 greater than the seismic moment, indicating a significant amount of ground movement was aseismic. The geodetic moment associated with the 1977 and 1983 rift zone events was about 0.8 x 10(26) dyn cm, nearly equal to the geodetic moment of the 1989 south flank earthquake. The 6-km depth of average slip during the earthquake was well above the 9-km depth of the base of the volcano, which indicates the average depth where strain was released by the 1989 earthquake was well within the volcanic edifice. The strain released by the earthquake, and the initial compression caused by rift zone intrusions, occurred over a range of depths, indicating the south flank behaves more as a large elastic spring coupled to a sliding mass rather than a rigid block sliding along the base of the volcano.